38 



Fishery Bulletin 103(1) 



— 400 



200 



between the 38 th and 39 th ocean circulus, the 

 circulus number 38.25 was assigned to that 

 boundary. We calculated the circulus spac- 

 ing in each interval as 4mm/Acirc, where 

 4mm = the width in mm of the interval, and 

 4circ = the difference between the interpo- 

 lated circulus numbers at the upper and 

 lower bounds of the interval. The circulus 

 spacing in each of the 50 intervals was aver- 

 aged across all the scales from the fish in a 

 group. This produced a profile of the average 

 spacing of circuli at 50 different positions 

 in relation to OE (lower bound of interval 

 1), the annulus (upper bound of interval 

 25) and the scale margin (upper bound of 

 interval 50). Finally, the group-average cir- 

 culus spacing in each of the 50 intervals was 

 plotted against the group-average radial dis- 

 tance from OE to the upper bounds of each 

 of the 50 intervals. For juvenile fish caught 

 in trawls in September 1999-2002, circu- 

 lus spacing was described at 25 intervals 

 in relation to OE (lower bound of interval 

 1) and the scale margin (upper bound of 

 interval 25). 



Seasonal changes in the spacing of circuli 

 at the growing edge of the scale may reflect 

 similar seasonal changes in the growth rate 

 of the juvenile and maturing coho salmon. 

 To investigate this possible correlation, we 

 measured the spacing of the last two circu- 

 lus pairs at the scale margin of juvenile fish 

 caught in early and late summer in 1982 

 and 1999 through 2002 and of maturing fish 

 caught in research nets 1981-83 and 2000-2002 and in 

 the ocean fisheries 1982-92 (Table 1). Mean spacing of 

 the last two circulus pairs was summarized by cruise 

 for the fish caught in research nets, and by 10-day catch 

 intervals for the fish caught in the ocean fisheries. The 

 seasonal trends in spacing at the scale margin were 

 then compared with the seasonal trend in apparent 

 growth rates of fish. 



Seasonal changes in fish growth rate 



Seasonal trends in growth rates of juvenile and matur- 

 ing coho salmon caught in research cruises 1981-83 and 

 1998-2002 were estimated from the changes between 

 cruises in average FL. We also estimated average growth 

 rates (pooled across years) of juvenile and adult coho 

 salmon during different seasons by fitting regressions 

 to the FL versus catch date data. 



Changing stock composition of the juvenile (Teel et 

 al., 2003) or maturing coho salmon caught in research 

 nets over the course of the summer could potentially 

 have a strong effect, independent of growth, on the size 

 distributions of fish caught at different times. Therefore, 

 changes over time in average FLs of mixed stocks of 

 fish, such as in our research collections, may not ac- 

 curately indicate actual fish growth rates. 



1000 



800 



600 - 



- 



o Adults, May-Sept. 1981-1983 

 ° Adults, June and Sept, 2001 , 2002 



• Adults, June 2000 and Table 2 



• Juveniles, 1981-1985, 1998-2001 



FL (mm) = 1 50-94 SR* 34.1 6, 

 n=2834. r 2 = 0.94 



Scale radius (mm) 



Figure 2 



Fork length (FL) versus scale radius (SR) for juvenile and matur- 

 ing coho salmon I O. kisutch) caught in research trawls and GM 

 regressions of FL versus SR fitted to juvenile and adult fish sepa- 

 rately. Note the allometry in the FL-SR relationship of juvenile 

 and adult fish. 



Because of the potential for error when inferring 

 seasonal changes in growth rate from changes over 

 time in average FLs of mixed stocks of fish, we also 

 examined temporal changes in FL of maturing CWT 

 coho salmon of known origin caught in the ocean hook- 

 and-line fisheries (sport and troll fisheries). Using data 

 available from the Pacific States Marine Fisheries Com- 

 mission 1 we investigated changes over the summer in 

 FLs of maturing CWT coho salmon originating from 

 six areas (north Oregon coast, lower Columbia River 

 basin-Oregon, lower Columbia River basin-Washington, 

 Willapa Bay basin, Grays Harbor basin, and the north- 

 west Washington coast). Because the date that a smolt 

 is released from a hatchery (e.g., March vs. June) could 

 affect its size the following year, we also grouped the 

 fish by release periods of 25-46 days duration. Da- 

 ta were available on FLs of maturing CWT fish from 

 1975-2002. For each group in each year we calculated 

 the average FL of CWT fish at 10-day intervals in the 

 hook-and-line fisheries (sport and troll fisheries) pooled 

 for all catch areas between California and Alaska. Data 

 were discarded when there were fewer than 5 fish mea- 



1 Regional Mark Information System CWT database (http:// 

 www.rmis.org). [Accessed on: 1 April 2003.1 



